目录

Optional

创建对象

安全消费值

安全获取值

ofElseGet

ofElseThrow

过滤

判断

数据转换

函数式接口

只有一个抽象方法的接口称为函数接口

常用默认方法

基本数据类型优化

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Optional

可以避免空指针异常<br/>

创建对象

  public static void main(String[] args) {Author author = getAuthors().get(0);Optional<Author> authorOptional = Optional.ofNullable(author);}

<br/>

安全消费值

  public static void main(String[] args) {Author author = getAuthors().get(0);Optional<Author> authorOptional = Optional.ofNullable(author);authorOptional.ifPresent(author1 -> System.out.println(author1.getName()));}

<br/>

安全获取值

不推荐使用get()

ofElseGet

获取数据并且设置为空时的默认值，为null就返回默认值

  public static void main(String[] args) {Author author = getAuthors().get(0);Optional<Author> authorOptional = Optional.ofNullable(author);Author author1 = authorOptional.orElseGet(() -> new Author());System.out.println(author1.getName());}

<br/>

ofElseThrow

为空就根据传入的参数进行异常抛出

  public static void main(String[] args) {Author author = getAuthors().get(0);Optional<Author> authorOptional = Optional.ofNullable(author);try {Author author1 = authorOptional.orElseThrow((Supplier<Throwable>) () -> {throw new RuntimeException("为空");});} catch (Throwable e) {throw new RuntimeException(e);}}

<br/>

过滤

不符合过滤条件返回的是新的Optional对象，其值为null

  public static void main(String[] args) {Author author = getAuthors().get(0);Optional<Author> author1 = Optional.ofNullable(author);author1.filter(author2 -> author2.getAge() > 18).ifPresent(author22 -> System.out.println(author22.getAge()));}

<br/>

判断

进行是否存在数据的判断，如果为空返回false，为true就不为空

  public static void main(String[] args) {Author author = getAuthors().get(0);Optional<Author> authorOptional = Optional.ofNullable(author);if (authorOptional.isPresent()) {System.out.println(authorOptional.get());}}

<br/>

数据转换

  public static void main(String[] args) {Author author = getAuthors().get(0);Optional<Author> authorOptional = Optional.ofNullable(author);authorOptional.map(au ->au.getBookList()).ifPresent(books -> System.out.println(books));}

<br/><br/>

函数式接口

只有一个抽象方法的接口称为函数接口

常用默认方法

and 相当于 &&找出年龄大于17并且长度大于1的作家

  public static void main(String[] args) {List<Author> authors = getAuthors();authors.stream().filter(new Predicate<Author>() {@Overridepublic boolean test(Author author) {return author.getAge() > 17}}.and(new Predicate<Author>() {@Overridepublic boolean test(Author author) {return author.getName() .length() > 1}})).forEach(author -> System.out.println(author.getAge()));}

or 相当于||打印年龄小于17的

  public static void main(String[] args) {List<Author> authors = getAuthors();authors.stream().filter(new Predicate<Author>() {@Overridepublic boolean test(Author author) {return author.getAge() > 17;}}.or(new Predicate<Author>() {@Overridepublic boolean test(Author author) {return author.getName() .length() > 1;}})).forEach(author -> System.out.println(author.getAge()));}

negate 相当于 !

  public static void main(String[] args) {List<Author> authors = getAuthors();authors.stream().filter(new Predicate<Author>() {@Overridepublic boolean test(Author author) {return author.getAge() > 17;}}.negate()).forEach(author -> System.out.println(author.getAge()));}

基本数据类型优化

数据量较大时，节约自动拆箱装箱的时间优化前

  public static void main(String[] args) {List<Author> authors = getAuthors();authors.stream().map(author -> author.getAge())}

优化后

  public static void main(String[] args) {List<Author> authors = getAuthors();authors.stream().mapToInt(author -> author.getAge())}